1. Field
The disclosed concept pertains generally to power electronic devices, such as multilevel power converters and, more particularly, to multilevel inverters and multilevel drives.
2. Background Information
A multilevel power inverter is a power electronic device that is structured to produce alternating current (AC) waveforms from a direct current (DC) input voltage. Multilevel power inverters are used in a wide variety of applications, such as, without limitation, variable speed motor drives and as an interface between a high voltage DC transmission line and an AC transmission line.
The general concept behind a multilevel power inverter is to use a number of power semiconductor switches coupled to a number of lower level DC voltage sources to perform power conversion by synthesizing a staircase voltage waveform. A number of different topologies for implementing a multilevel power inverter are well known, including, but not limited to, a neutral point clamped (NPC) topology, a flying capacitor (FC) topology, and an H-bridge topology. There is also a neutral point piloted (NPP) topology as disclosed herein.
As is conventional, a bank of capacitors (a “DC link”) coupled to one or more DC voltage inputs is often used to provide multiple DC voltage sources employed for operation of a multilevel power inverter. For example, it is known to use such a DC link comprising a bank of capacitors in the NPC and FC topologies.
A conventional layout for a multilevel NPC power converter or a multilevel NPP power converter leads to a relatively large number of crossing electrical connections and, thus, a relatively very complicated electrical bus structure. A complicated layered buswork can be cost prohibitive in medium voltage equipment. In low voltage power converters, circuit layout complexity also adds cost.
It is known to employ relatively simpler topologies or relatively costly laminated buswork containing relatively many layers of insulators and conductors. In low voltage equipment, for example, a relatively highly complex interconnected circuit often leads to multilayer printed circuit boards (PCBs).
FIG. 1 shows a conventional electrical layout of a five-level NPC topology 2 for a single phase-leg having an AC output 4, five DC inputs 6, 8, 10, 12, 14, and a plurality of control inputs 16, 18, 20, 22 and 24, 26, 28, 30. For example, three of these topologies 2 are used to make up a three-phase DC to AC inverter (not shown). The mechanical layout (not shown) of the five-level NPC topology 2 follows from this structure of electrical connection. This structure extends to any number of levels, where a string of diodes (not shown, but see, e.g., diode strings 32, 34, 36 shown vertically with respect to FIG. 1), an extra capacitor (not shown, but see, e.g., capacitors 38, 40, 42, 44), and two extra IGBT devices (not shown, but see, e.g., IGBTs 46, 48, 50, 52 and 54, 56, 58, 60) are added for each additional level. The control and gating circuit for this phase-leg and its control inputs 16, 18, 20, 22 and 24, 26, 28, 30 is not shown.
FIG. 7 shows a conventional electrical layout of a five-level NPP topology 70 for a single phase-leg. For example, three of these single phase-legs are used to make up a three-phase DC-AC NPP inverter (not shown). NPP inverters are marketed by Converteam Inc. of Pittsburgh, Pa. The mechanical layout (not shown) of the five-level NPP topology 70 follows from this structure of electrical connection. This structure extends to any number of levels. The control and gating circuit for this phase-leg and its control inputs is not shown.
FIG. 11 shows a conventional electrical layout of a three-level NPP topology 80 for a single phase-leg where there are three DC voltage inputs 82, 84, 86.
FIG. 13 shows a conventional electrical layout of a four-level NPP topology 90 for a single phase-leg where there are four DC voltage inputs 92, 94, 96, 98.